Conventionally, laminated films having an ethylene-vinyl alcohol copolymer layer have been used in applications such as packaging materials for food, medical applications and the like, taking advantages of their superior gas barrier properties, thermoformability and the like. Recently, for the purpose of improving various types of performances such as gas barrier properties, various multilayered structures have been proposed in which a plurality of resin-layers are laminated, each layer having a thickness of micron or submicron order.
Conventional multilayered structures developed so far in which a plurality of resin-layers of an ethylene-vinyl alcohol copolymer are laminated include, for example: (1) elastomeric barrier films in which layers of a microlayer polymer composite constituted with a barrier material such as an ethylene-vinyl alcohol copolymer, and an elastomer material such as thermoplastic polyurethane are laminated (see, for example, Patent Documents 1 and 2); (2) multilayer films alternately including layers of a hard polymer material such as an ethylene-vinyl alcohol copolymer and a plastic polymer material (see, for example, Patent Document 3), and the like.
However, according to the aforementioned conventional multilayered structure (1), inter layer adhesiveness between a barrier material such as an ethylene-vinyl alcohol copolymer and an elastomer material such as thermoplastic polyurethane is not considered in particular. In addition, also according to the aforementioned conventional multilayered structure (2), the hard polymer material per se such as an ethylene-vinyl alcohol copolymer, and combination of the same with the plastic polymer material, and the like are not investigated in connection with the inter layer adhesiveness, but only a technique of strengthening the adhesiveness between each layer using a joining layer merely consisting of a hot melt adhesive is disclosed. Therefore, according to these conventional multilayered structure (1) and multilayered structure (2), the inter layer adhesiveness is insufficient, and due to delamination between layers and the like, cracks are likely to be generated on the barrier layer, whereby durability may be deteriorated. As a result, gas barrier properties are insufficient according to the conventional multilayered structures (1) and (2) in applications for which superior gas barrier properties are required, leading to the disadvantages of difficulty in use.
On the other hand, inner faces of pneumatic tires are provided with an inner liner layer constituted with a butyl rubber having low gas-permeability such as a butyl rubber or a halogenated butyl rubber as a principal component is conventionally provided for preventing air leakage and maintaining the air pressure of the tire at a constant level. However, when the content of such a butyl rubber is increased, the strength is decreased due to an unvulcanized rubber, whereby cleavage of the rubber, perforation of the sheet, and the like are likely to be caused. Particularly, when gauge reduction of the inner liner is carried out, disadvantages of exposure of the code provided on the inner face being likely to occur in manufacturing tires are caused.
Therefore, the content of the butyl rubber is limited per se, and when a rubber composition containing the butyl rubber is used, it is necessary for the inner liner layer to have a thickness of about 1 mm in light of air barrier properties. Therefore, the mass of the inner liner layer in the tire accounts for about 5%, thereby hampering improvement of the fuel efficiency of automobiles by decreasing the weight of tires.
Thus, taking into consideration social demands for energy saving in recent years, for the purpose of reduction in weight of automobile tires, a procedure for the gauge reduction of inner liner layers has been proposed. In exemplary procedures developed as such a procedure, a nylon film layer or a vinylidene chloride layer is used as an inner liner layer, in place of conventional butyl rubbers (for example, see Patent Documents 4 and 5). In addition, a procedure in which a film of a composition containing a thermoplastic resin such as a polyamide resin or a polyester resin blended with an elastomer is used as an inner liner layer was also developed (for example, see Patent Document 6).
However, although the weight of tire can be reduced to some extent according to such a method in which these films are used, the method may be accompanied by a disadvantage of inferior crack resistance and/or resistance to fatigue from flexing when used at low temperatures of no greater than 5° C. in particular as compared with the case of commonly used butyl rubber blend composition layers, due to the matrix being a crystalline resinous material, and manufacturing of tires additionally involves complicated steps.
On the other hand, ethylene-vinyl alcohol copolymers are superior in gas barrier properties as described above. Since the ethylene-vinyl alcohol copolymer has an air permeation rate of no greater than one hundredth of that of inner liner rubber compositions containing a butyl rubber, a capacity of maintaining internal pressure can be significantly improved even if the thickness is no greater than 50 μm, whereby reduction in weight of tires is enabled. Therefore, it is efficacious to use an ethylene-vinyl alcohol copolymer for inner liners of tires in order to reduce air permeability of pneumatic tires, and thus a pneumatic tire having a tire inner liner constituted with an ethylene-vinyl alcohol copolymer was developed (for example, see Patent Document 7).
However, although a great effect of improving the capacity of maintaining internal pressure may be achieved in the case in which the ethylene-vinyl alcohol copolymer is used as an inner liner, due to a significantly high modulus of elasticity as compared with rubbers commonly used for tires, leading to inferior flex resistance, fracture and/or crack may be generated as a result of deformation upon flexion. Accordingly, in the case in which an inner liner formed from an ethylene-vinyl alcohol copolymer is used, a capacity of maintaining internal pressure before using the tire is greatly improved; however, the capacity of maintaining internal pressure may be reduced in the tire after use subjected to flexion deformity during rotation of the tire, as compared with before use thereof.
In order to solve the problem, an inner liner for an inner face of tires produced using a resin composition constituted with, for example, 60 to 99% by weight of an ethylene-vinyl alcohol copolymer having an ethylene content of 20 to 70 mol % and a degree of saponification of no less than 85%, and 1 to 40% by weight of a hydrophobic plasticizer was developed (for example, see Patent Document 8); however, flex resistance of the inner liner is not necessarily satisfactory.
Therefore, development of a multilayered structure which can be suitably used for inner liner for pneumatic tires, and the like, has been desired that has high-level flex resistance while maintaining superior gas barrier properties, and that allows for reduction in thickness.